1. Field of the Invention
The present invention relates to a continuous method of manufacturing chlorine-free tetraalkoxysilanes from metallic silicon powder and alcohol in the presence of a liquid, basic catalyst mixture with improved selectivity and improved space-time yield.
2. Description of the Background
Methods of continuous production of tetraalkoxysilanes are known, and essentially two methods are used in industry: The first method is the direct reaction of silicon powder with alcohol, while the second method is the esterification of tetrachlorosilane with alcohol.
The latter method, esterification of tetrachlorosilane with alcohol, can be conducted at normal pressure and without energy input in simple reactors. The ester mixture which is produced must be distilled, however, in order to remove, in particular, chlorine-containing impurities such as chlorine-containing esters and dissolved hydrogen chloride. The cost of such distillation is very high if one wishes to obtain a product tetraalkoxysilane of low chlorine content. For example, such distillation is conducted in a column having a large number of plates, in countercurrent distillation with alcohol. The residual chlorine in the tetraalkoxysilane product is neutralized in an additional reaction stage. The salt thereby obtained must be separated and appropriately discarded, and the tetraalkoxysilane must undergo additional distillative purification. Further, the esterification of tetrachlorosilane with alcohol involves the liberation of gaseous hydrogen chloride, which is costly to purify if it is to be economically recovered. The method essentially requires two reaction stages to convert the silicon to tetraalkoxysilane, viz. synthesis of the chlorosilane, and esterification of the chlorosilane. For more efficiency there is a need for a method employing only one reaction stage.
A method of producing alkoxysilanes in a single step reaction is the direct synthesis from silicon and alcohols. In U.S. Pat. No. 5,084,590, such a direct synthesis in the presence of a copper catalyst is described. However, the predominant product of this reaction is the trialkoxysilane. Where the objective of the direct synthesis is tetraalkoxysilanes, there is no known way of optimizing the reaction conditions which appreciably avoids formation of Si--H bonds and thereby formation of trialkoxysilanes. Accordingly, yields of tetraalkoxysilanes are unsatisfactory. Moreover, costly processing of the copper-containing residues is required, rendering the copper-catalyzed synthesis unsuitable for economical and efficient manufacture of tetraalkoxysilanes.
German 28 16 386 describes a catalytic method of manufacturing tetraalkoxysilanes from silicon and alcohols without the introduction of heavy metals such as copper. The catalysts comprise alkali alcoholate, ether alcohols and alkanolamines. Preferably, the method is conducted in a stirred vessel. The yield does not exceed 90 wt. %, based on the silicon introduced, which is not sufficiently high for an efficient continuous process. A side reaction of hydrogen with the alcohols occurs, forming water and alkanes, which necessitates a costly distillation with a scavenger, e.g., azeotropic distillation, in order to remove the water. The water present deactivates the catalyst and also leads to undesirable oligomerization of the tetraalkoxysilanes.
U.S. Pat. No. 4,288,604 describes a discontinuous method of reacting silicon and alcohols to form tetraalkoxysilanes in the presence of a basic catalyst. The silicon used must be comminuted under a nitrogen atmosphere in order to avoid blocking of the silicon surface with oxygen. In order to enable initiation of the reaction, inert, high-boiling solvents are employed. In addition, hydroxide scavengers must be employed in order to deactivate hydroxyl groups which are formed or which are introduced with reaction mixture components. Despite the use of appreciable amounts of inert solvents, hydroxide scavengers, and alkali alcoholates (as catalysts), the method is unsuitable for a continuous process, because it falls far short of the necessary high conversions of the silicon and ethanol to product. A need, therefore, continues to exist for a continuous process of producing tetraalkoxysilanes in high yield.